Components made of light metal, in particular aluminum alloys, are provided with surface protection by anodising, often also referred to as eloxing, in order to make them resistant to chemical influences. A known standard process for this is chromic acid anodising (CAA). In the production of aircraft, a large number of small sheet metal parts, for example from 10 000 to 14 000 per day, are treated by chromic acid anodising. To carry out anodising, the parts are held in support frames by means of clamps in batches of, for example, up to 800 and dipped simultaneously into appropriate electrochemical baths. Since the components have to be provided with electrical contacts and have a voltage applied to them in order to be anodised, it can happen that individual parts are not anodised or incorrectly anodised because of contact problems. This can be the case for, for example, 0.1% of the parts. Since parts which have not been anodised do not represent the desired in-specification state, these have to be recognized. The recognition of unanodised parts does not represent a problem in the case of the abovementioned chromic acid anodising since anodised parts have a “grey” appearance while unanodised parts have a “bright” appearance.
Since chromic acid anodising is problematical in terms of its environmental acceptability, alternative anodising processes are desirable. The so-called TSA process (TSA=tartaric sulphuric acid) is such an alternative anodising process. As regards the corrosion protection to be provided, the TSA process is at least as good as the above-described CAA process, but the anodised layer cannot be detected by means of simple visual inspection. The visual inspection to be employed in a simple manner for the CAA process can thus not be employed for the more environmentally friendly TSA process to detect unanodised parts.